Radiotherapy is a process whereby a beam of harmful radiation is directed generally towards a region of a patient, usually in order to treat a tumour within that region. The radiation causes damage to living cells in its path, and hence inhibits or reduces the tumour. It also damages healthy tissue if applied in significant doses, and therefore efforts are made to limit the dose to healthy tissues while maintaining the prescribed dose to cancerous tissue.
One apparently straightforward means of limiting the dose to healthy tissue is to direct the beam towards the tumour from a plurality of different directions. Thus, the total dose delivered to the tumour can be significantly greater than the dose applied to any individual section of surrounding tissue. A common approach to doing so is to mount the radiation source on a rotatable support, with the source being oriented towards the rotation axis of the support so that the beam intersects with the axis. Thus, as the support rotates, the beam always passes through the point of intersection (usually referred to as the “isocentre”) but does so from every radial direction around the isocentre. This requires the support to be rotated around the patient; the support has a significant mass and therefore the engineering challenge that this presents is significant.
Another means of limiting the dose applied to healthy tissue is the so-called “multi-leaf collimator” or “MLC” as shown in, for example, EP-A-314,214. An plurality of long narrow leaves are arranged side-by side in an array, and are individually controllable via a servo-motor so that they can each be extended or retracted by a desired amount. Thus, by moving individual leaves, a collimator can be made to a desired shape. A pair of such collimators, one either side of the beam, allows the beam to be shaped as desired thereby allowing healthy tissue to be placed in shadow.
In a multi-leaf collimator, the leaves are generally thin in the direction transverse to the direction of movement, to provide a good resolution, and long in the direction of movement so as to provide a good range of movement. In the direction of the beam, the leaves need to be relatively deep; even when made of a high atomic number material such as Tungsten, such depth is required in order to offer an adequate attenuation of the beam. Thus, leaves are relatively heavy and difficult to move.
Both of these aspects of a radiotherapy apparatus require the relevant geometry item (in this case the rotatable support and the MLC leaves) to be moved during treatment in an accurate manner. Older “step and shoot” methods called for the geometry item to be moved to a specific location, which can be checked easily by known servo-control methods. However, to improve treatment times, more modern treatment control methods call for the geometry item to be moved at a specific (linear or rotational) speed over a specific time period, after which it is moved at a (potentially) different speed for a further time period. This raises the issue of inertia.
Specifically, a conventional treatment plan might (for example) call for the geometry item to move at a particular speed v1 over a time period t1 followed by a speed v2 over a subsequent time period t2. The geometry items cannot and will not change their speed immediately, there will in practice be a catch-up period during which the actual speed will be incorrect, either too high if v1>v2 or too low if v1<v2. In either case, the geometry item will be at an incorrect location during delivery of at least part of the dose. Our earlier application US 2009-121155-A1 therefore provided a radiotherapeutic apparatus comprising a geometry item that was moveable to adjust the geometry of the beam, and a control unit being arranged to cause variations in the speed of movement of the geometry item and also adjust the dose rate of the radiation source for a period of time after a change in the speed of the geometry item. This sought to compensate for the effects of inertia by restraining the dose rate temporarily, under local control.